1. Field of the Invention
The present invention relates to a method for decomposing plastic.
2. Prior Art
Conventionally, plastic has such problems that it is burnt to generate toxic gas or to have incomplete or partial combustion. Burning plastic recently becomes hard to be performed due to and in view of energy-saving policies and global warming. Besides, burying plastic in the ground cannot find any extra spaces for the purpose while causing a problem of environment pollution.
It is most preferable (and quite hard) to reuse (recycle) plastic under the foregoing circumstances. Nowadays, reusing plastic is mainly realized in such manners as crushing and melting again to form fibers or blocks, or introducing plastic wastes into a furnace.
Another approach for reusing plastic has been hitherto proposed as that plastic is pyrolytically decomposed to become low-molecular and be reused. In detail, plastic is heated more than heat decomposition temperature (without oxygen, the same in the following) so as to cause high-molecular carbon-carbon bonding to be cut and become low-molecular plastic as gasoline or naphtha.
The crushing method for reusing plastic involves a quite troublesome crushing operation using a crusher or the like to make smaller plastic wherein plastic when mixing foreign objects, such as metal, causes malfunction of machine. Resultant crushed objects that are at least few millimeters in size can usually not be melted again, as they are, and molded. The crushed objects may be mixed with concrete to form blocks. However, the crushed objects are large in size, and plastic and cement have poor adhesiveness between them, resulting in that blocks themselves are low in strength and cannot be usable equally to blocks provided conventionally.
The foregoing method introducing plastic wastes into a furnace has such problem that the resultant melt when formed into a usual molded article shows poor quality, which is inferred to be caused by an admixture or a mixed foreign object. Moreover, thermoplastic that has cross-linking to some extent when molded is not usable to be mixed with a virgin material and remolded.
In burning plastic for disposal, it is said that burning at 300 to 600° C. causes dioxin. And plastic does not burn under 300° C. Hence, plastic is to be burnt at high temperatures, but burning plastic at high temperatures is hard to be performed by plastic itself and needs a large quantity of fuel uneconomically.
Burning plastic at high temperatures can avoid the problem of dioxin but generates a great deal of carbon dioxide due to burning of a large quantity of fuel (and plastic itself) contrary directly to the national policies reducing carbon dioxide in respect of global warming.
The method pyrolytically decomposing plastic for providing low molecular plastic employs high heating temperatures (at least 600° C., and 800° C. at maximum) and thereby shows a poor efficiency of using energy. Moreover, heating pieces of plastic to those temperatures is hard. A method of heating plastic merely placed in a container cannot transmit heat efficiently to plastic due to much air and tends to heat in excess of necessity, having large energy loss. Using heavy oil for the heating operation is problematic in respect of energy saving and prevention of global warming.
Furthermore, there is concern about generation of dioxin depending on kinds of plastic and the degree of heating.
An approach heating pieces of plastic by mixing pieces of plastic with heated sand in order to improve heating efficiency has been proposed recently. This method can raise the heating efficiency but does perform heating pieces of plastic to the pyrolysis temperature, which does therefore not provide an essential solution of the foregoing problems.